Transparent conductors are widely used in the flat-panel display industry to form electrodes that are used to electrically switch light-emitting or light-transmitting properties of a display pixel, for example in liquid crystal or organic light-emitting diode displays. Transparent conductive electrodes are also used in touch screens in conjunction with displays. In such applications, the transparency and conductivity of the transparent electrodes are important attributes. In general, it is desired that transparent conductors have a high transparency (for example, greater than 90% in the visible spectrum) and a low electrical resistivity (for example, less than 10 ohms/square).
Transparent conductive metal oxides are well known in the display and touch-screen industries and have a number of disadvantages, including limited transparency and conductivity and a tendency to crack under mechanical or environmental stress. Typical prior-art conductive electrode materials include conductive metal oxides such as indium tin oxide (ITO) or very thin layers of metal, for example silver or aluminum or metal alloys including silver or aluminum. These materials are coated, for example, by sputtering or vapor deposition, and are patterned on display or touch-screen substrates, such as glass. For example, the use of transparent conductive oxides to form arrays of touch senses on one side of a substrate is taught in U.S. Patent Application Publication No. 2011/0099805 entitled “Method of Fabricating Capacitive Touch-Screen Panel”.
Transparent conductive metal oxides are increasingly expensive and relatively costly to deposit and pattern. Moreover, the substrate materials are limited by the electrode material deposition process (such as sputtering) and the current-carrying capacity of such electrodes is limited, thereby limiting the amount of power that is supplied to the pixel elements and the size of touch screens that employ such electrodes. Although thicker layers of metal oxides or metals increase conductivity, they also reduce the transparency of the electrodes.
Apparently transparent electrodes including very fine patterns of conductive elements, such as metal wires or conductive traces are known. For example, U.S. Patent Application Publication No. 2011/0007011 teaches a capacitive touch screen with a mesh electrode, as do U.S. Patent Application Publication No. 2010/0026664, U.S. Patent Application Publication No. 2010/0328248, and U.S. Pat. No. 8,179,381, which are hereby incorporated in their entirety by reference. As disclosed in U.S. Pat. No. 8,179,381, fine conductor patterns are made by one of several processes, including laser-cured masking, inkjet printing, gravure printing, micro-replication, and micro-contact printing. In particular, micro-replication is used to form micro-conductors formed in micro-replicated channels. The apparently transparent micro-wire electrodes include micro-wires between 0.5μ and 4μ wide and a transparency of between approximately 86% and 96%.
Conductive micro-wires formed in micro-channels embossed in a substrate, for example as taught in CN102063951, which is hereby incorporated by reference in its entirety. As discussed in CN102063951, a pattern of micro-channels are formed in a substrate using an embossing technique. Embossing methods are generally known in the prior art and typically include coating a curable liquid, such as a polymer, onto a rigid substrate. A pattern of micro-channels is embossed (impressed or imprinted) onto the polymer layer by a master having an inverted pattern of structures formed on its surface. The polymer is then cured. A conductive ink is coated over the substrate and into the micro-channels, the excess conductive ink between micro-channels is removed, for example by mechanical buffing, patterned chemical electrolysis, or patterned chemical corrosion. The conductive ink in the micro-channels is cured, for example by heating. In an alternative method described in CN102063951, a photosensitive layer, chemical plating, or sputtering is used to pattern conductors, for example using patterned radiation exposure or physical masks. Unwanted material (such as photosensitive resist) is removed, followed by electro-deposition of metallic ions in a bath.
In general, individual micro-wires are intended to be invisible to a user so that very narrow micro-wires are helpful. The micro-wires typically include metals that are not transparent but have a width of only a few microns, so that they are imperceptible to the unaided human eye. Because the micro-wires are so narrow, they have a tendency to break or crack.
Multi-layer conductive micro-wires are known in the art, for example co-pending U.S. patent application Ser. No. 13/779,917, filed Feb. 28, 2013 entitled Multi-Layer Micro-Wire Structure, by Yau et al and to commonly-assigned co-pending U.S. patent application Ser. No. 13/779,939, filed Feb. 28, 2013 entitled Making Multi-Layer Micro-Wire Structure, by Yau et al; the disclosures of which are incorporated herein.
Thick films of conductive material are used to form conductors on substrates, for example for printed circuit boards using etching or screen printing. These methods generally do not form very thin and very narrow conductors that are invisible to a viewer or that are constrained to a limited area on a substrate.
Electro-plating methods are also known to form conductors. In particular, electroless plating techniques rely on an autocatalytic process that deposits metals on a seed layer from a solution in which the seed layer is immersed. Such seed layers are typically very thin (only a few microns thick) and are not substantially electrically conductive. The metal deposited on the seed layer is relatively thick compared to the seed layer and much more electrically conductive. However, the time required to plate a sufficiently electrically conductive thick metal layer on a seed layer is often quite long, for example 20 or 30 minutes. This length of time is problematic in a commercial, high-volume manufacturing process. Furthermore, the process of plating increases the substrate area rendered opaque by the seed layer and plated layer, rendering the layers more visible. Thus, it is difficult to form very narrow, very long, and very conductive micro-wires in a high-volume manufacturing process using such methods.